avoiding light through a glass
I believe you need to rethink what you are really looking for. If you want the refraction part to be actually transparent in the image (have a lower alpha value), you can leave out the glass node entirely. Instead, mix a transparent shader with a glossy shader using a fresnel type of factor. This isn't a physically correct phenomenon anyway, so no need to get all crazy about how to set it up: just something that looks decent is needed.

(This image is plenty high-res enough to see everything clearly even though it doesn't show up correctly here for some reason. Just open it up in a new tab in your browser to see all of the settings or refer to the .blend file.)



In this blend file, the "transparent" looking areas are actually transparent in the render.

Here is the file that I used for the demo: